Known instrumentation for providing analysis of electromagnetic radiation X-ray sources typically utilize either silicon charge coupled devices (CCD) or lithium drift detectors. Both of these devices have well recognized shortcomings in their capability to provide imaging and energy resolution of impinging radiation, especially in their ability to detect higher energy X-ray sources.
Recently, an X-ray detector adapted to operate with X-ray sources in the 1 to 30 Kev range has been developed which is capable of providing both high spatial resolution along with good energy resolution. In this device, fully described in U.S. Pat. No. 4,472,728 to Grant et al, a thick silicon body has an embedded matrix, or grid, of aluminum thermally migrated therethrough. The aluminum matrix defines the walls of a rectangular array of X-ray detector cells or pixels. A thermally migrated aluminum electrode is also formed centrally through each of the cells with biasing means being connected to the aluminum cell walls and the centralized aluminum electrode for causing lateral charge carrier depletion between the cell walls so that incident X-ray energy causes a photoelectric reaction within the silicon producing collectible charge carriers in the form of electrons or holes which are collected and used for imaging.
While such detector can perform its intended function, several limitations have been noted in the manner of its fabrication and operation. The walls of the grid defining the pixel must be as thin as possible, i.e., no more than 2 mils in width. Unfortunately, such narrow walls are extremely difficult to drive through a wafer using conventional thermal migration techniques and require long drive-in times to complete. Further, the thermally migrated walls have a tendency to drift randomly off the vertical axis. This is due to a variety of factors including difficulty in migrating certain shapes, crystal imperfections, nonuniform temperatures and wafer position in the thermomigration oven. Attempts to reduce the drive-in times required by thermomigrating points, separated from each other by a mil, rather than continuous lines, resulted in a different problem. A contact metal, such as aluminum, would have to be employed to complete the circuit between the thermomigrated points. If it overlapped the wall boundary, which it would invariably do due to the random lateral drift in the wall boundary, the junction of the walls and the substrate would be shorted out therby rendering the device inoperative. Additionally, while it is relatively simple in subsequent processing steps to align the 4-6 mil central electrode, alignment of the thin walls has been found to be quite difficult.
In actual usage of the detector, leakage currents were higher than expected. This has been diagnosed to be transistor action of charge injection due to the arrangement of a p-type aluminum central electrode separated from the p-type aluminum walls by an n-type silicon body, forming a pnp type transistor.